Our overall aim is to describe in the mammalian system the contribution of maternal RNA and protein stored in the egg to embryogenesis, in particular the role of maternal ribosomes and messenger RNAs in early protein synthesis and the possible role of proteins encoded in maternal messengers or present in the egg in early differentiation. The experiments depend in part on labeling RNA or protein of ovulated mouse ova by exposure of growing oocytes to labeled precursors in vivo, and recovery and characterization of labeled RNA or protein in the ovulated egg and early embryo. We propose to continue the analysis of egg ribosomes, and the retention of poly (A) ion RNA in developing embryos. New studies are proposed to determine whether maternal RNA enters the embryonic nuclei during early development and to relate the loss of maternal RNA to differentiation of the trophoblast by autoradiography. Stable proteins synthesized at different stages of oocyte growth will be characterized, both with respect to specific proteins and if possible the spectrum of abundant proteins. An attempt will be made to demonstrate egg mRNA activity by translation in vitro and to characterize the products. Finally, the program of gene expression in growing oocytes will be clarified by measuring quantitatively the accumulation of ribosomal RNA or total RNA during growth.